As Russ pointed out 300.4(f) and 250.92(b) are the correct sections for this. Bear in mind (with respect to 300.4(f) when you're using a manufactured connector where the edge of the fitting is flared or rounded (i.e a 2" squeeze connector for SEU cable)you don't need a fiber or plastic bushing. If it's threaded conduit entering directly into the enclosure...you do.
Also, (with respect to 250.92(b)a bonding locknut or bushing is required if you have concentric knockouts because it is possible that the integrity of the metal parts between the enclosure and the knockout layers can become separated either at the time of installation or over time from vibration.
I used to use plastic bushings on all squeeze connectors. Someone also once told me that on all connectors 2" and above it was mandatory. However, if you look closely at the 2" SEU squeeze connector it does have a rounded edge and cannot cut into the wire. I haven't used plastic bushings in several years now nor have any of my collegues in the contractors' association and I haven't failed an inspection.
The next service upgrade I do I'll ask the inspector and get an opinion from him. If I find out differently I'll post it.
I think the issue of conduit size must have been in the code at one time. When I was a cub I was taught conduits 1-1/4? and larger required bushings. The fact is it is biased on a conductor size of #4 and larger. You can have a 4? conduit with number 6 AWG wire and not need a bushing if you have connectors with smooth rounded edges such as EMT connectors.